1. Field of the Invention
The present invention generally relates to the production techniques of integrated circuits and, more particularly, to a dynamically switched voltage screening method for quality assurance testing of integrated circuit dies on a wafer.
2. Description of Related Art
Integrated circuit (IC) chips or dies fabricated on the same wafer have a wide range of performance in critical parameter characteristics due, in a large part, to process variations. As a result, out-going quality and reliability is compromised by the inconsistent performance of IC chips from the same wafer lot. Typically, stress testing is performed on the wafer during this fabrication process to eliminate the weaker chips from entering the next phase of the production cycle. For example, one of the tests regarded as one of the most severe, is that of subjecting the devices to particularly high temperatures; typically between 85° C. and 150° C., for accelerating the infant mortality of chips in a wafer. This test, commonly referred to as “burn-in”, has the objective of stimulating the failure of those devices which have developed some defects during the fabrication process and/or during handling. However, this test does not segregate parts based solely on performance characteristics. Rather, the stress is uniformly applied to all chips of a semiconductor wafer. Additionally, this test is time consuming, equipment intensive, and costly to perform.
Alternatively, the industry has administered a voltage stress test for implementing a screen with less degrading effects. A voltage screen is basically a higher than normal voltage applied during wafer test that effectively causes defects to manifest as failures in IC chips during subsequent verification or operational testing. The problem associated with applying a higher than normal voltage level across a semiconductor die is that some dies processed with short channel lengths have a higher tendency to fail when exposed to higher voltages. These short channel length devices would not otherwise be failures except for their vulnerability to high voltage exposure. The failures are not related to defects, rather, to the over stressing of the short channel lengths in the die. Thus, as applied, the voltage screen can be responsible for false failures, an undesirable quality assurance test result. Nevertheless, the industry standard has been to continuously apply a voltage stress at one voltage level to all die on a semiconductor wafer.
As discussed by Lee and Sonoda, in “TEST SYSTEM FOR NARROWING THE RANGE OF PERFORMANCE CHARACTERISTICS OF MONOLITHIC INTEGRATED CIRCUITS”, IBM Technical Disclosure Bulletin, Vol. 15, No. 4, September 1972, some circuits, such as those employed with FET technology, have performance characteristics that can differ by as much as 100% due to tolerances in the threshold voltage. Because of these wide differences, there are at one end of the distribution curve chips exhibiting fast response time and high power dissipation, and at the other end of the distribution curve chips having slower circuit response time and lower power dissipation. Thus, sorting the wafer at the IC chip level during quality assurance or reliability testing would be advantageous to identifying the more vulnerable short channel length devices and exposing them to less stress, thus, eliminating false failures during voltage screening.
In the prior art, parts have been screened during wafer testing to classify individual IC chips at various speeds. In U.S. Pat. No. 5,196,787 issued to Ovens et al. on 23 Mar. 1993, entitled, “TEST CIRCUIT FOR SCREENING PARTS”, a test circuit was developed on the die to measure the DC characteristics of a device, which in turn, enabled one to estimate the AC characteristics. The AC characteristic estimations were then used to screen parts into various speed classes. However, no suggestion is made to dynamically switch or adjust the stress test levels based on the operational parameters measured.
Another method for determining the operational speed of an IC chip is disclosed in U.S. Pat. No. 5,099,196 issued to Longwell et al. on 24 Mar. 1992, entitled, “ON-CHIP INTEGRATED CIRCUIT SPEED SELECTION.” By forming an oscillator in an IC semiconductor chip to generate pulses representative of the speed of other components formed in the chip, the operational speed of the oscillator (typically, a ring oscillator), and therefore, that of the other components formed in the semiconductor chip, can be determined. Again, the stress test levels are not altered in response to the operation speed measurements taken.
IC chip segregation tests also include bit pattern recognition on each device under test. This method is particularly useful in memory device testing. In U.S. Pat. No. 4,335,457 issued to Early on 15 Jun. 1982, entitled, “METHOD FOR SEMICONDUCTOR MEMORY TESTING”, semiconductor memory devices are tested using a special purpose computer which employs simple test patterns to determine the weakest bits of the device, and then tests only these relatively few “weak bits” along with structurally and operationally adjacent bits using highly complex test patterns to determine if the device is functioning properly. Bit pattern recognition, however, is not a stress test screen. Thus, no adjustment of stress test levels, predicated on the bit pattern results, is either taught or suggested by this prior art.
Still, other methods may be employed to distinguish the IC chips based on variations in the operational parameters. However, independent of the method chosen, some functionally operating IC chips continue to remain vulnerable to excessive stress test screening levels due to chip-to-chip process variations.
Bearing in mind the problems and deficiencies of the prior art, it is therefore an object of the present invention to provide an apparatus and method that determines the speed of IC chips on a semiconductor wafer and adjusts the stress test levels based on the speed measured for each device.
It is another object of the present invention to provide an apparatus and method for effectively protecting the short channel IC chip population with a lower voltage during voltage stress testing of a semiconductor wafer.
A further object of the invention is to increase the outgoing quality and reliability of a semiconductor die using a voltage screen without falsely rejecting the short channel die during voltage stress testing of a semiconductor wafer.
It is yet another object of the present invention to provide an apparatus and method for segregating IC chips capable of a higher voltage withstand level without compromising the resultant yield from the wafer lot.
Another object of the present invention is to increase the measure of reliability of the devices on a semiconductor wafer by assigning supply current limits as a function of device speed.
Still other objects of the invention will in part be obvious and will in part be apparent from the specification.